The performance of many applications benefit from being implemented over service provider networks that support multipoint network services. A multipoint network service is one that allows each customer edge (CE) end point or node to communicate directly and independently with all other CE nodes. Ethernet switched campus networks are an example of a multipoint service architecture. The multipoint network service contrasts with more traditional point-to-point services, such as hub-and-spoke network services, where the end customer designates one CE node to the hub that multiplexes multiple point-to-point services over a single User-Network Interface (UNI) to reach multiple “spoke” CE nodes. In a hub-and-spoke network architecture, each spoke can reach any other spoke only by communicating through the hub. Traditional network service offering to the end customers via wide area networks (WANs) such as Frame Relay (FR) and asynchronous transfer mode (ATM) networks are based on a hub-and-spoke service architecture.
Virtual Private Network (VPN) services provide secure network connections between different locations. A company, for example, can use a VPN to provide secure connections between geographically dispersed sites that need to access the corporate network. There are three types of VPN that are classified by the network layer used to establish the connection between the customer and provider network. Layer 1 VPNs are simple point-to-point protocol (PPP) connections such as leased lines, ISDN links, and dial-up connections. In a Layer 2 VPN (L2VPN) the provider delivers Layer 2 circuits to the customer (one for each site) and provides switching of the customer data. Customers map their Layer 3 routing to the circuit mesh, with customer routes being transparent to the provider. Many traditional L2VPNs are based on Frame Relay or ATM packet technologies. In a Layer 3 VPN (L3VPN) the provider router participates in the customers Layer 3 routing. That is, the CE routers peer only with attached PEs, advertise their routes to the provider, and the provider router manages the VPN-specific routing tables, as well as distributing routes to remote sites. In a Layer 3 Internet Protocol (IP) VPN, customer sites are connected via IP routers that can communicate privately over a shared backbone as if they are using their own private network. Multi-protocol label switching (MPLS) Border Gateway Protocol (BGP) networks are one type of L3VPN solution. An example of an IP-based Virtual Private Network is disclosed in U.S. Pat. No. 6,693,878. U.S. Pat. No. 6,665,273 describes a MPLS system with a network device for traffic engineering.
Virtual Private LAN Service (VPLS) is an emerging technology that addresses the need for Layer 2 multipoint VPN that connects multiple sites within a specific metropolitan geographic area. VPLS is an architecture that delivers a Layer 2 multipoint VPN service that in all respects emulates an Ethernet LAN across a wide metropolitan geographic area. All services in a VPLS appear to be on the same LAN, regardless of location. In other words, with VPLS, customers can communicate as if they were connected via a private Ethernet segment, i.e., multipoint Ethernet LAN services. VPLS thus supports the connection of multiple sites in a single bridged domain over a managed IP/MPLS network.
In typical VPLS architecture with an IP/MPLS service provider (SP) network core, the CE devices are connected to the service provider network via a PE device. (The connection between a CE-PE pair of devices is commonly referred to as a UNI.) Each PE-CE pair is shown connected by an Attachment Circuit (AC). An AC is the customer connection to a service provider network; that is, the connection between a CE and its associated PE. An AC may be a point-to-point connection on a physical interface, a PPP session from an L2TP tunnel, an MPLS Label Switched Path (LSP), or a virtual port, and may be any transport technology, i.e., Frame Relay, ATM, a VLAN, etc. In the context of a VPLS, an AC is typically an Ethernet port, in which Ethernet serves as the framing technology between the CE device and the PE router. CE devices can also be connected through several edge domains, also known as access domains, which are interconnected using an MPLS core network. Such access domains can be built using Ethernet switches and techniques such as VLAN tag stacking (so-called “QinQ” encapsulation). By way of example, each PE device in an access domain typically includes a Virtual Switch Instance (VSI) that emulates an Ethernet bridge (i.e., switch) function in terms of MAC address learning and forwarding in order to facilitate the provision of a multi-point L2VPN. In such networks, pseudowires (PWs) are commonly utilized to connect pairs of VSIs associated with different access domains.
A PW is a virtual connection between two PE devices which connect two ACs. Conceptually in context of the VPLS service, a PW can be thought of as point-to-point virtual link for each offered service between a pair of VSIs. Therefore, if each VSI can be thought of as a virtual Ethernet switch for a given customer service instance, then each PW can be thought of as a virtual link connecting these virtual switches over a Packet Switched Network (PSN) to each other for that service instance. During setup of a PW, the two connecting PE devices exchange information about the service to be emulated in order to be able to properly process packets received from the other end in the future.
Another type of provider provisioned VPN architecture that uses PWs is the Virtual Private Wire Service (VPWS). VPWS is a Layer 2 service that provides point-to-point connectivity (e.g., Frame Relay, ATM, point-to-point Ethernet) and can be used to create port-based or VLAN-based Ethernet private lines across a MPLS-enabled IP network. Conceptually, in the context of the VPWS service, a PW can be thought of as a point-to-point virtual link connecting two customer ACs. After a PW is setup between a pair of PEs, frames received by one PE from an AC are encapsulated and sent over the PW to the remote PE, where native frames are reconstructed and forwarded to the other CE. PEs in the SP network are typically connected together with a set of tunnels, with each tunnel carrying multiple PWs. The number of PWs setup for a given customer can vary depending on the number of customer sites and the topology for connecting these sites.
Similar to Ethernet switches, VPLS-capable PE devices are capable of dynamically learning the Media Access Control (MAC) addresses (on both physical ports and virtual circuits) of the frame packets they replicate and forward across both physical ports and PWs. That is, each PE device is capable of learning remote MAC addresses-to-PW associations and also learns directly attached MAC addresses on customer facing ports. To achieve this result, PE devices maintain a Forwarding Information Base (FIB) table for each VPN and forward frames based on MAC address associations. Another attribute of an Ethernet network is that frames with unknown destination MAC addresses are flooded to all ports.
For an Ethernet network to function properly, only one available path can exist between any two nodes. To provide path redundancy and prevent undesirable loops in the network domain topology caused by multiple available paths, Ethernet networks typically employ Spanning Tree Protocol (STP), or some variant of STP, e.g., MSTP or RSTP. (For purposes of the present application, STP and its variants are generically denoted by the acronym “xSTP”.) Switches in a network running STP gather information about other switches in the network through an exchange of data messages called Bridge Protocol Data Units (BPDUs). BPDUs contain information about the transmitting switch and its ports, including its switch and port Media Access Control (MAC) addresses and priorities. The exchange of BPDU messages results in the election of a root bridge on the network, and computation of the best path from each switch to the root switch. To provide path redundancy, STP defines a tree from the root that spans all of the switches in the network, with certain redundant paths being forced into a standby (i.e., blocked) state. If a particular network segment becomes unreachable the STP algorithm reconfigures the tree topology and re-establishes the link by activating an appropriate standby path. Examples of networks that run STP are disclosed in U.S. Pat. Nos. 6,519,231, 6,188,694 and 6,304,575.
A particular redundancy problem arises when Ethernet and STP are combined with pseudowires. Basically, when there are two or more pseudowires connecting different Ethernet access domains that independently run STP, broadcast and multicast packets can be replicated, and packets can be “looped back” across the core network through the pseudowires. The source of this problem is twofold: On one hand, STP is designed to build a path with no loops by disabling (i.e., blocking) any links which could forward traffic to the same destination. On the other hand, VPLS and Ethernet Relay Service (ERS) applications, which use VLAN tags to multiplex several non-same-destination pseudowires to a single port, assume that a full mesh of PWs connecting all involved PEs is the most efficient network topology. (Loops are dealt with in VPLS and ERS via a mechanism known as “split-horizon”.)
One possible solution to this problem is to devise a mechanism for running STP over pseudowires; however, this approach is considered too unwieldy and difficult to implement. Another proposed architectural solution is to utilize only a single PW that connects different Ethernet access domains across the core network. The primary drawback of this latter approach is that it means that it installs a single point of failure in network connections. In other words, if the PW connection fails or if the associated PE devices in the access networks fail, end-to-end connectivity is defeated.
Thus, there is an unsatisfied need for alternative network architectures and topologies that overcomes the shortcomings of the prior art.
By way of further background, U.S. Pat. No. 6,073,176 discloses a multi-chassis, multi-link point-to-point protocol (PPP) that uses Stack Group Bidding Protocol (SGBP) to conduct multi-link PPP sessions for links that either originate or terminate on different systems. Historically, SGBP has been used for dial-up customer (UNI) facing interfaces to allow network servers to be stacked together and appear as a single server, so that if one server fails or runs out of resources, another server in the stack can accept calls. For instance, U.S. Pat. No. 6,373,838 teaches a dial-up access stacking architecture (DASA) with SGBP that implements a large multi-link dial port in which multiple communication links from one site are established to stack group members that operate together as a multi-link bundle.